Sp100 nuclear antigen is an interferon stimulated antigen found in the bile duct of primary biliary cirrhosis. Histologically sp100 'dots' regions of the cell nucleus. Viral infection and mitogens affect the expression of the Sp100 autoantigen. Cells grown in the presence of interferons (α, β, and γ) revealed an increase both in size and number of the Sp100 protein-containing nuclear dots and increase the protein concentration. This raises "the question whether cytokine-mediated increase of Sp100 protein expression plays a role in induction of anti-Sp100 autoantibodies."Some sp100 variants contain a domain similar to two interferon-inducible nuclear phosphoproteins, suppressin and DEAF1. This defines a novel protein motif, the HNPP-box. Another class of variants has high mobility group 1 (HMG1) protein sequence as a domain.